The present invention is especially applicable to the repositioning of spacers in a nuclear reactor, such as a CANDU® reactor. In a CANDU® nuclear reactor, the pressure tubes which contain the fuel bundles are each positioned within a calandria tube. It is necessary to have an annular space maintained between the pressure tube and the calandria tube to allow for the circulation of gases which thermally insulate the hot pressure tube from the relatively colder calandria tube and the heavy water moderator which flows in the space outside the calandria tube.
The annular space is maintained by annulus spacers, which are one component that make up a CANDU® reactor fuel channel. These spacers maintain the radial spacing between two coaxial tubes, an inner pressure tube and an outer calandria tube, and help the calandria tubes support the inner pressure tubes. There are both loose-fitting and snugfitting annulus spacers, which differ in design.
A loose-fitting spacer comprises a closely coiled spring made from a square cross section wire, assembled on a circular girdle wire to form a torus. The girdle wire of the loose-fitting spacer is welded to form a continuous loop of fixed size. The minor diameter of the loose-fitting spacer is such that it is slightly larger than that of the outside diameter of a pressure tube. As such, the spacer fits loosely around the pressure tube. The spacer stays in its installed position by friction alone and not by spring tension. Loose-fitting spacers were used in earlier CANDU® reactors.
A snug-fitting spacer comprises a closely coiled spring made from a square cross section wire, assembled on a circular girdle wire to form a torus. The girdle wire is not welded, therefore the effective minor diameter of the spacer can be increased by applying tension to extend the coiled spring. The design of the snug-fitting spacer is such that the coil spring is under some tension when installed on a pressure tube, resulting in a snug fit. The design of the annulus spacer is such that they are not fixed rigidly in position. The spacer is held in position by spring tension and friction. Snug fitting spacers typically maintain their initial desired position, however, it may be possible that a spacer may move from its desired position, or, during the course of operation of a reactor, it may be desirable to move the position of a spacer.
Typically, four spacers are used in a fuel channel, each spacer being positioned at a different axial position. To provide the required support, the annulus spacers must be located at the proper position; if a spacer is out of position, the hot pressure tube may come into contact with the cooler calandria tube. Such contact between the inner pressure tube and the outer calandria tube is unacceptable.
During installation of spacers in such a reactor, or, as suggested above, during its operation, spacers may be displaced from their required positions with the result that the pressure tubes will lack the necessary configuration of supports to carry the distributed load in operation of the reactor, and serious problems may arise from sagging of these tubes. It is therefore desirable to have some way of detecting and repositioning (if necessary) the spacers after installation or even after the reactor has been operating for some time. The optimal position of a spacer may change slightly during the operating life of a reactor. The original installed spacer position is based on the support conditions throughout the reactor life. However, it may be desirable to reposition the spacers late in the reactor life to better suit the end of life conditions. Repositioning spacers late in life may extend the operating life of a reactor by some years, resulting in a significant economic benefit.
These annulus spacers are located between the pressure tubes and the calandria tubes and are not directly accessible by mechanical means. Since the spacer position is not fixed mechanically, it is desirable to have a means to detect their position.
U.S. Pat. No. 4,613,477 (“U.S. '477) discloses a method for repositioning garter springs, used as annulus spacers between the coolant tubes and calandria tubes of fluid cooled nuclear reactors. Such garter springs are not directly accessible by mechanical means. In the method of U.S. '477, an electromagnetic coil is advanced along the selected fuel channel to a position adjacent the garter spring, and a current pulse is passed through the coil thereby to exert an electromagnetic repulsive force on the garter spring having a component in the direction of the required displacement. This technique is applicable to the loose-fitting spacers which have the welded girdle wire. The welded girdle wire of the loose-fitting spacer forms a continuous electrical circuit that is necessary for the electromagnetic-based technique. The electromagnetic technique does not work on the tight-fitting spacer, because the non-welded girdle wire does not provide a continuous electrical path within the spacer.
A need remains for an apparatus and method for detecting and repositioning tight-fitting annulus spacers.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.